Photorefractive device having an electro-optical material between two photoconductive materials

ABSTRACT

A photorefractive apparatus is exposed to a write beam or radiation for creating a system of interference fringes in the apparatus, as well as to a read beam or radiation which diffracts the system created and includes at least one elementary pattern (1) having an electrooptical material (6) with a high electrooptical coefficient with respect to the read radiation, which is transparent to the read radiation and which is surrounded by photoconductive materials (2, 4). The photoconductive materials are transparent to the read radiation and under the effect of the write radiation are able to respectively produce electrons and holes having a high mobility. Application to the optical processing of signals.

BACKGROUND OF THE INVENTION

The present invention relates to a photorefractive apparatus, i.e. anapparatus where a photorefractive effect is liable to occur.

Explanations of the photorefractive effect are provided in:

(1) Photorefractive effect, David Pepper, Jack Feinberg, NicolaiKukhtarev, For the Science, No. 158, December 1990, pages 58 to 64.

Reference should also be made to:

(2) Topics in Applied Physics, Springer Verlag, Vol. 61, Photorefractivematerials and their applications, Volume 1, Chapter 8, ThePhotorefractive Effect in Semiconductors, Alastair M. Glass andJefferson Strait.

The apparatus according to the invention can be used as a rewritableholographic recording apparatus.

As a function of the components of the apparatus according to theinvention, said rewritable holographic recording apparatus can besensitive in the visible range or in the near infrared range.

The present invention has, inter alia, as applications all those of arewritable holographic recording apparatus and in particular thedeflection of light beams, the formation of reconfigurable opticalinterconnections, the production of phase conjugation mirrors and theprovision of signal optical treatment or processing devices, which canbe of an analog or digital nature.

In the case of analog signals, the present invention e.g. applies to theproduction of optical correlation devices and in the case of digitalsignals to the production of arrays of optical logic gates.

For some years now, photorefractive materials have constituted the mostwidely investigated class of rewritable holographic recordingapparatuses for all dynamic holography applications.

The principle of the photorefractive effect is as follows. The chargecarriers are photoexcited under the effect of a non-homogeneousillumination such as an interference pattern and are non-homogeneouslyredistributed, thus creating a space charge field. The latter induces bythe electrooptical effect a variation of the refractive index, which isthe "image", apart from a possible spatial displacement, of the initialinterference pattern. The two fundamental elements of thephotorefractive effect are consequently photoconductivity and theelectrooptical effect.

Most frequently, the electrooptical effect is the Pockels effect and itis then necessary to use photorefractive materials in the form of solidmonocrystals, which can be in the form of a parallelepiped, whereof eachof the edges is a few millimeters.

Compared with several other non-linear optical effects, thephotorefractive effect has an original characteristic. Thus, it issensitive to light energy per surface unit and not to the lightintensity of the write radiation.

This is due to the integration effect inherent in the formation of thespace charge field, the number of displaced charges per time unit beingproportional to the photon flux.

Thus, the variation of the refractive index dN increases, in a timecalled the response time and which is designated "tau", which isinversely proportional to the incident light intensity until it reachesa limit value dNmax.

Known photorefractive materials can be classified in two categories fromthe performance standpoint:

slow materials having a significant effect on saturation, such asLiNbO₃, BaTiO₃, KNbO₃, (BaSr)Nb₂ O₆ (SBN),

fast materials having a limited effect on saturation, such as sillenites(Bi₁₂ SiO₂₀, Bi₁₂ GeO₂₀, Bi₁₂ TiO₂₀) and semiconductors (GaAs, InP,CdTe, GAP).

When account is taken of the refractive index variation per lightintensity unit dN/I, which is a function of the figure of merit N³·r/eps (in which I,N,r and eps respectively represent the intensity ofthe write radiation, the refractive index of the material, the effectivelinear electrooptical coefficient of the material and the dielectricconstant of said material), it can be seen that all known solidphotorefractive materials have identical characteristics to within afactor of 10.

The interest of the materials GaAs, InP and CdTe is of having asignificant sensitivity to the wavelengths used in the field of opticaltelecommunications.

However, the known photorefractive materials suffer from a disadvantage.It is not possible to find materials which simultaneously have a shortresponse time to optical excitation and a high refractive indexvariation dN.

Various solutions have been tried for solving this problem.

1. Application of an electric field for increasing the space chargefield.

This has been successful with sillenites and semiconductors. However, itleads to a certain increase (10 to 100) of the time constant comparedwith operation in a zero field and particularly leads to the appearanceof a photocurrent, as a result of the application of an electric field,limits the maximum intensity which can be withstood by thephotorefractive crystal and therefore the response time cannot be madeas short as when no field is applied.

2. Application of an electric field for increasing the effectiveelectrooptical coefficient.

If the use wavelength is close to that corresponding to the width of theforbidden band of the material, the electrorefraction (Franz-Keldysheffect) becomes high and can lead to an effective electroopticalcoefficient (proportional to the field) which is several times higherthan the linear electrooptical coefficient.

This resonant effect (unlike the standard photorefractive effect),combined with the space charge field increase effect, makes it possibleto achieve the highest limit values dNmax in solid semiconductors.

However, the "speed" limitations due to the photocurrent remain the sameas in the case referred to in 1, i.e. the response time to an opticalexcitation remains relatively high.

3. The use of a multiple quantum well structure.

It is known that the electrorefraction effect can be significantlyincreased on passing from a solid material to a multiple quantum wellstructure. This has applications in the field of light modulation inguided configuration or perpendicular to the substrate. Reference can bemade to the following document in this connection:

(3) Resonant photodiffractive effect in semiinsulating multiple quantumwells, D. D. Nolte, D. H. Olson, G. E. Doran, W. H. Knox and A. M.Glass, J. Opt. Soc. Am.B, Vol. 7, No. 11, November 1990, pages 2217 to2225.

In connection with the photorefractive effect, document (3) hasdemonstrated the possibility of obtaining significant diffractionefficiencies (through 1 micrometer of active material). The methodproposed in (3) consists of making semiinsulating a GaAs/AlGaAs multiplequantum well structure by introducing protons.

Compared with the methods described hereinbefore, the electric field isonly used for increasing the effective electrooptical coefficient andnot for a contribution to the displacement of the charges in themultiple quantum well structure. Therefore, the advantages of the methoddescribed in (1) are lost, but it is possible to use short responsetimes.

Another interest of the multiple quantum well structure compared with asolid structure is the possibility of adjusting the operating wavelengthfor a given application.

However, the diffraction efficiencies obtained with the method describedin (3) are approximately 10⁻⁵ and are therefore too low for theapplications envisaged hereinbefore in connection with rewritableholographic recording apparatuses.

In order to obviate this disadvantage, one possible solution consists ofincreasing the thickness of the active material but, in this case, thegrowth method and more particularly the implantation method areunsuitable and it is therefore necessary to seek an optimized multiplequantum well structure for these electrooptical effects and containing adeep center with an energy level and a concentration which are carefullychosen, which can make optimization difficult.

Thus, with known photorefractive materials, it is impossible tosimultaneously have a high speed for establishing the refractive indexsystem, said speed being more particularly linked with the mobility muof the charge carriers and a high amplitude for the photoinduced indexsystem, said amplitude being linked with the electrooptical figure ofmerit value N³ ·r/2.

Thus, the fastest of the known materials is InP:Fe (mu approximately3000 cm² /V·s) and systems can be recorded by the photorefractive effectin semiconductors such as GaAs, InP and CdTe in a few dozen picosecondswith luminous fluxes of approximately 1 mJ/cm² at 1.06 micrometer.However, the figure of merit of InP is only 25 pm/V.

Conversely, certain materials having a perovskite structure have afigure of merit of approximately 1000 pm/V but with mobilities wellbelow 0.1 cm² /V·s.

It is hardly likely that it will be possible to find a photorefractivematerial simultaneously having the necessary speed and high amplitudereferred to hereinbefore.

In the field of low light intensities (approximately 10 mW/cm²), whichare e.g. supplied by diode-pumped Nd:YAG lasers, it is possible to applya continuous electric field of approximately 10 kV/cm to InP in order topartly compensate the weakness of the electrooptical coefficient (theresponse times are under these conditions approximately 50 to 100 ms),but the current increase in the presence of high intensities requiredfor lowering the response time, although only 1 microsecond, makes itimpossible to use this process in the case of high light intensities.

SUMMARY OF THE INVENTION

The present invention solves the problem of finding a photorefractivestructure or apparatus having both a high speed of establishing therefractive index system therein during an appropriate opticalexcitation, i.e. a short response time to said excitation and a highamplitude of said index system, i.e. a large index variation.

The present invention therefore relates to a photorefractive apparatusfor exposure to a write radiation for creating a system of interferencefringes in the apparatus and to a read radiation which diffracts thesystem created, characterized in that it comprises at least oneelementary pattern, each elementary pattern having an electroopticalmaterial at a high electrooptical coefficient compared with the readradiation and which is transparent to the latter and a firstphotoconductive material and a second photoconductive material, whichsurround the electrooptical material and which are transparent to theread radiation and which, under the effect of the write radiation, areable to respectively produce electrons having a high mobility and holeshaving a high mobility.

Thus, by illuminating one face of the apparatus with the write beam, aperiodic electric field is created in the first and secondphotoconductive materials and said field induces a refractive indexvariation in the electrooptical material of the apparatus.

The term high electrooptical coefficient is understood to mean anelectrooptical coefficient at least equal to 500 pm/V. The term "highmobility" means a mobility of at least 10 cm² /V·s.

According to a special embodiment of the apparatus according to theinvention, the first photoconductive material is a semiconductortransparent to the read radiation and having in its forbidden band, apartly occupied energy level which, under the effect of the writeradiation, mainly emits electrons, having a high mobility, towards theconduction band of said semiconductor (type n photoconductor), and thesecond photoconductive material is a semiconductor transparent to theread radiation and having, in its forbidden band, a partly occupiedenergy level which, under the effect of the write radiation, mainlyemits holes, having a high mobility, towards the valence band of thesemiconductor constituting the second photoconductive material (type pphotoconductor).

Preferably, the energy levels respectively corresponding to the firstand second photoconductive materials are deep levels. The apparatus isthen easier to use with low exciting light intensities.

In a special embodiment, the first and second photoconductive materialsare made from a same semiconductor material with different dopingsmaking the first photoconductive material of type n and the secondphotoconductive material of type p.

Preferably, the thickness of the electrooptical material is of the sameorder of magnitude as the spacing of the interference fringes. Thisthickness can be approximately 0.5 to 1.

The electrooptical material can have a multiple quantum well structure.

As a variant, the electrooptical material can be in the form of a singlelayer.

The apparatus according to the invention can comprise a plurality ofelementary patterns.

Finally, each elementary pattern is preferably separated from anadjacent elementary pattern by an insulating layer, which is transparentto the read radiation and which constitutes an electrical screen betweenthe respective photoconductive materials of said elementary patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein:

FIG. 1 is a diagrammatic view of an apparatus according to the inventionhaving a single elementary pattern and whose electrooptical material hasa multiple quantum well structure.

FIG. 2 diagrammatically shows an electron emission to the conductionband of the semiconductor material constituting the firstphotoconductive material of the apparatus of FIG. 1.

FIG. 3 diagrammatically shows an emission of holes to the valence bandof the semiconductor material constituting the second photoconductivematerial of the apparatus of FIG. 1.

FIG. 4 diagrammatically shows an apparatus according to the inventionhaving a plurality of elementary patterns, each of the latterincorporating an electrooptical material having a multiple quantum wellstructure.

FIG. 5 diagrammatically shows another apparatus according to theinvention having a single elementary pattern and whose electroopticalmaterial is in the form of a single layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus according to the invention diagrammatically shown in FIG.1 has a single elementary pattern 1. During its use, it is exposed to awrite radiation or to an exciting radiation, which creates a system ofinterference fringes in the apparatus and whereof it is possible to seethe two interfering write radiations Ri1 and Ri2.

When the apparatus is exposed to the write radiation or following saidexposure, a read radiation R1 or use radiation is supplied to theapparatus.

The elementary pattern 1 has two photoconductive layers 2, 4 and anelectrooptical multi layer 6 having a multiple quantum well structureand which is positioned between the two photoconductive layers 2, 4. Themultilayer 6 has a high electrooptical coefficient with respect to theread beam and is transparent to the latter. The layers 2, 4 aresemiconductor layers, whose respective majority carriers have a highmobility and which are transparent to the read radiation.

The semiconductor material of the layer 2 is a type n photoconductor andmore specifically has in its forbidden band a deep energy level which ispartly occupied and which, under the effect of the write radiation,mainly emits electrons to the conduction band of said semiconductormaterial.

This is diagrammatically illustrated in FIG. 2, in which BV1, BC1 andNP1 respectively represent the valence band, the conduction band and thedeep level relative to the semiconductor layer 2, the arrow F1symbolizing the electron emission from the deep level NP1 to theconduction band BC1. In conventional manner, the axis E represents theaxis of the electronic energies.

The semiconductor material of the layer 4 is a type p photoconductor andmore specifically has in its forbidden band a deep energy level, whichis partly occupied and which, under the effect of the write radiation,mainly emits holes towards the valence band of said semiconductormaterial of layer 4.

This is diagrammatically illustrated in FIG. 3, in which BV2, BC2 andNP2 respectively represent the valence band, the conduction band and thedeep level of the semiconductor material of the layer 4.

The arrow F2 in FIG. 3 symbolizes the passage of electrons from thevalence band BV2 to the level NP2 (i.e. the emission of holes from thedeep level NP2 to the valence band BV2) under the effect of the excitingradiation or beam.

During the use of the apparatus of FIG. 1, onto one of the layers 2 and4 (layer 2 in the case of FIG. 1) and parallel to a plane Pperpendicular to said layer, are supplied the two write radiation beamsRi1 and Ri2. These two radiation beams interfere in the apparatus andmore specifically create an interference pattern in each of the layers2, 4.

Moreover, it is general practice to operate in transmission and the notshown substrate on which are stacked the layers 2, 4 and 6 is thenchosen so as to be transparent to the read radiation.

In addition, the multilayer 6 having a multiple quantum well structure,reverse polarization takes place of the apparatus of FIG. 1, the layer 2being brought to a negative potential compared with the layer 4.

In FIG. 1, the strata shown in both layers 2 and 4 symbolize theinterference pattern formed. In FIG. 1, the symbols +++ and ---symbolize the redistribution of charges as a result of the formation ofsaid interference pattern. Thus, for each of the layers 2 and 4, theadjacent strata are allocated with opposite charges and two stratarespectively belonging to the layers 2 and 4 and facing one anothercarry opposite charges.

As a result of the creation of the interference pattern, whose spacingis designated L, the electrooptical material constituted by themultilayer 6 is subject to a periodic electric field along an axis Zperpendicular to the layers 2 and 4, said field also being periodicalong the axis X, which is perpendicular to the axis Z and parallel tothe plane P.

The amplitude of the electric field is approximately equal to:

    Ed=(2pi/L)·(k·T/e)

in which pi, k, T and e respectively represent the number equallingapproximately 3.14, the Boltzmann constant, the temperature of theapparatus and the electron charge (the systems of electrical chargesinduced in the layers 2 and 4 are, by design, in phase opposition).

These considerations apply if the thickness of the multilayer 6 is ofthe same order of magnitude as the spacing of the interference fringesand which is e.g. approximately 1 micrometer.

Thus, when the spacing L of the interference fringes is approximately 1micrometer, it is possible to choose a thickness of approximately 1micrometer for the multilayer 6.

By repeating the elementary pattern of FIG. 1 about 100 times, asatisfactory interaction length is then obtained of approximately 100micrometers.

The present invention and in particular the apparatus shown in FIG. 1,differs from the structure proposed in document (3) by a separation ofthe photoconduction and electrooptical functions, which leads to abetter overall optimization.

Moreover, the thickness increase of the apparatus, by stacking severalelementary patterns, is relatively easier than a thickness increase ofthe structure as proposed in (3), as a result of the lack of need toinstall deep centers by proton bombardment.

The multilayer 6 forming the electrooptical material of the apparatus ofFIG. 1 is e.g. the multiple quantum well structure resulting from thestacking of 15 patterns InGaAsP/InP 7 nm/25 nm (whose total thickness is1.3 micrometre).

It is known from the following document:

(4) "Quaternary quantum wells for electrooptic intensity and phasemodulation at 1.3 and 1.55 micrometer" J. E. Zucker, I. Bar-Joseph, N.I. Miller, V. Koren and D. S. Chemla, Applied Physics Letters, Vol. 54,No. 1, January 1989, pp. 10 to 12.

That the said multiple quantum well structure has an electroopticalfigure of merit comparable to BaTiO₃ when working at a wavelengthcorresponding to the exciton resonance and by applying an electricalfield of 100 kV/cm a value of 800 pm/V was obtained in the vicinity of1.3 micrometers.

Three examples are given below for the layers 2 (n type doping) and 4 (ptype doping):

(a) layer 2 of InP:Ti and layer 4 of InP:Fe,

(b) layer 2 of InP:Fe (with a high concentration of |Fe⁺⁺ | and F⁺⁺ andlayer 4 of InP:Fe,

(c) layer 2 of InP:Cr (donor) and layer 4 of InP:Cr (acceptor).

Thus, it is possible to use the same semiconductor material withdifferent doping types for producing the layers 2 and 4.

It is also possible to use two different semiconductor materials, onewith a n type doping for producing the layer 2 and the other with a ptype doping for producing the layer 4.

FIG. 4 diagrammatically shows an apparatus according to the inventionhaving a plurality of elementary patterns of the type shown in FIG. 1.More specifically, the apparatus of FIG. 4 comprises, on a substrate 8,a plurality of elementary "layer 4-multilayer 6-layer 2" patternsstacked on one another, so that a layer 4 is in contact with thesubstrate 8.

Preferably, each elementary pattern is separated from the followingpattern by a layer 10 transparent to the read radiation R1 and whosethickness is e.g. a few hundred nanometers and which forms an electricalshield between said patterns, so that the electrical field produced inthe layer 2 of one elementary pattern is not compensated by theelectrical field produced in the layer 4 of the adjacent elementarypattern,

The apparatus of FIG. 4 also comprises a contact layer 12 covering thelayer 2 furthest from the substrate 8.

FIG. 4 also shows electrical contacts 14 located on the edge of theapparatus and one of the contacts 14 is located on the contact layer 12and the other contact 14 on an edge of the substrate 8, the apparatusbeing produced in such a way that said edge of the substrate 8 is notcovered by the layers stacked on the substrate.

The apparatus of FIG. 4 is polarized by appropriate polarization means16 permitting a reverse polarization thereof, the contact layer 12 beingraised to a positive potential with respect to the substrate.

In a purely indicative and in no way limitative manner the substrate 8is of n-doped InP, each layer 10 is of n⁻ -doped InP, the contact layer12 is of p-doped InP, each layer 2 is of n-doped InP, each layer 4 is ofp-doped InP and each layer 6 is an InGaAs/InP or InGaAsP/InP multiplequantum well structure.

The apparatus of FIG. 4 can optionally have another, not shown n⁺ -dopedInP contact layer between substrate 8 and the layer 4 closest to thelatter.

Thus, it is possible to produce an apparatus according to the inventionwith layers 2 and 4 made from InP and multilayer 6, which constitutemultiple quantum well structures compatible with InP, e.g. InGaAsP/InPor InGaAs/InP structures.

The layers necessary for producing such apparatuses can be produced bymethods known in connection with such materials.

It is in particular possible to use the MOCVD method and the MBE methodwith gas sources lending themselves to the production of multiplequantum well structures and InP:Fe or InP:Ti semiinsulating layers, withgrowth rates of several micrometers per hour.

Details are given hereinafter of the performance characteristicsobtainable with apparatuses according to the invention.

1. Performance characteristics relative to the variation of therefractive index dNmax obtained with a lambda reading wavelength, therefractive index N then being in the form

    N=No+dNmax·cos(2 pi·x/lambda)

Using the example of an electrooptical material having a multiplequantum well structure with a high electrooptical coefficient, it hasbeen shown hereinbefore that the order of magnitude of the periodiccomponent of the field applied to this material is approximately 1kV/cm. This leads to a value of approximately 10⁻⁴ for dNmax, withelectrooptical coefficients of approximately 1000 pm/V.

An evaluation of the diffraction efficiency Rd is as follows:

    Rd=(pi·Lt·dNmax/lambda).sup.2

in which Lt represents the total active thickness of the apparatus. Ifthe latter is 100 micrometers (stacks of 100 elementary patterns), thediffraction efficiency is 10⁻³, which is completely suitable fordifferent applications, e.g. in the image proceessing field.

It should be noted that this estimate of the diffraction efficiency doesnot take account of the effect of the absorption system, which developsin addition to the refractive index system and which increases the totaldiffraction efficiency.

The estimate made hereinbefore assumes that the use wavelength isslightly below that associated with the gap of the electroopticalmaterial (layer 6).

A choice of a use wavelength slightly higher than that associated withsaid gap is also possible and leads to at least 10 times higher dNmaxvalues. However, in this case, it is necessary to reduce the number ofelementary patterns due to the absorption.

Thus, the choice of the "use wavelength-number of elementary patterns"pair is to be optimized as a function of the characteristics of thematerials, the technological possibilities and applications.

2. Performance characteristics relative to the photosensitivity dN/I.

The interest of the apparatus according to the invention, which makes itpossible to separate the photoconduction function from theelectrooptical function is particularly apparent on considering thephotosensitivity, because the overall figure of merit N³ ·r/eps is wellabove those of the known photorefractive materials (the product N³ ·rhere being associated with the high electtrooptical coefficientmaterial, e.g. a material having a multiple quantum well structure andthe quantity eps being associated with the photoconductive material ormaterials having a low dielectric constant, e.g. InP).

The order of magnitude of the photosensitivity gain (taking as areference solid InP in non-resonant operation) ranges from 50 (for a usewavelength below that corresponding to the gap) to a few hundred (for ause wavelength just above that corresponding to the gap).

It is pointed out that the displacement of the charges in the materialsurrounding the multiple quantum well structure is not assisted by acontinuous electrical field and that consequently the space charge fieldformation time constant is identical to that of a solid materialoperating without an applied field.

3. Performance characteristics relative to the response time tau.

It has been shown hereinbefore that an apparatus according to theinvention provided a significant photosensitivity gain compared with theknown photorefractive materials. This makes it possible to move back thelimits which, in practise, result from the heating due to opticalabsorption and the Joule effect. It should also be noted that the use ofthin layers is advantageous from the heat dissipation standpoint.

The use of an electrical field perpendicular to the apparatus and havinga value of approximately 100 kV/cm (to be compared with the value ofapproximately 10 kV/cm typically used in solid photorefractivesemiconductors) could give rise to the idea that serious heatingproblems due to the Joule effect could occur.

However, in the present invention, the situation is radically differentbecause, apart from the fact that thin layers can be used, theintermediate electrooptical material has a higher resistivity and is notnecessarily photoconductive at the use wavelength (it is in factpreferable that this is not the case).

Moreover, it is possible to use an operating method where the electricalfield (used solely for obtaining a high electrooptical coefficient) isapplied just after a brief, high intensity writing pulse, the intensityof the read radiation being lower.

The present invention makes it possible to obtain response times ofapproximately 1 ns, the light intensity necessary for reaching suchvalues being approximately 1 to 100 kW/cm².

It is pointed out that as a result of the symmetry of revolution withrespect to the perpendicular for the different layers of an apparatusaccording to the invention, the result obtained is independent of thepolarization of the write radiations of the hologram, when said beamsare polarized. Obviously, in this case, the polarizations of the writebeams must be identical in order to produce an interference pattern.

In the present invention, the intermediate material with a highelectrooptical coefficient does not necessarily have a multiple quantumwell structure. It is possible to use a "bulk" material with a highelectrooptical coefficient (e.g. approximately 10⁴ pm/V).

In this connection, it is possible to use a ferroelectric materialhaving a high electrooptical coefficient, e.g. BaTiO₃.

FIG. 5 shows an elementary pattern corresponding to this possibility. Asingle BaTiO₃ layer 18 is located between an e.g. amorphous Siphotoconductive layer 20 and another e.g. amorphous Si photoconductivelayer 22.

A thickness of 1 micrometer for the intermediate or active layer 18 thenleads to a diffraction efficiency of approximately 10⁻⁵.

Another apparatus according to the invention is obtained by stackingelementary patterns, of the type described relative to FIG. 5. A stackof ten elementary patterns, whereof the active layers have in each casea thickness of 1 micrometer, then leads to a diffraction efficiency ofapproximately 10⁻³.

We claim:
 1. Photorefractive apparatus which is to be exposed to a writeradiation to create a system of interference fringes in the apparatusand to a read radiation which is diffracted by the created system, saidapparatus comprising one elementary pattern, said elementary patternhaving an electrooptical material which has a high electroopticalcoefficient with respect to the read radiation and which is transparentto said read radiation and a first n-type photoconductive material and asecond p-type photoconductive material which surround the electroopticalmaterial and which are transparent to the read radiation and which,under the effect of the write radiation, are able to respectivelyproduce electrons having a high mobility of at least 10 cm² /V·S andholes having a high mobility of at least 10 cm² /V·S, saidelectrooptical coefficient being at least equal to 500 pm/V. 2.Apparatus according to claim 1, wherein the first photoconductivematerial is a semiconductor transparent to the read radiation andhaving, in a forbidden band of said first photoconductive material, apartially occupied energy level which, under the effect of the writeradiation, mainly emits electrons having a high mobility of at least 10cm² /V·S, towards the conduction band of said semiconductor, and whereinthe second photoconductive material is a semiconductor transparent tothe read radiation and having, in a forbidden band of said secondphotoconducting material, a partially occupied energy level which, underthe effect of the write radiation, mainly emits holes having a highmobility of at least 10 cm² V·S, towards the valence band of saidsemiconductor constituting the second photoconductive material. 3.Apparatus according to claim 2, wherein the energy levels correspondingrespectively to the first and the second photoconductive materials aredeep levels.
 4. Apparatus according to claim 2, wherein the first andthe second photoconductive materials are formed from the samesemiconductor material, whose doping is of the n type for the firstphotoconductive material and of the p type for the secondphotoconductive material.
 5. Apparatus according to claim 1, wherein thethickness of the electrooptical material is of an order of magnitudewhich is the same as an order of magnitude of the spacing of theinterference fringes.
 6. Apparatus according to claim 1, wherein thethickness of the electrooptical material is approximately 0.05 to 1micrometre.
 7. Apparatus according to claim 1, wherein theelectrooptical material has a multiple quantum well structure. 8.Apparatus according to claim 1, wherein the electrooptical material isin the form of a single layer.
 9. Photorefractive apparatus which is tobe exposed to a write radiation to create a system of interferencefringes in the apparatus and to a read radiation which is diffracted bythe created system, said apparatus comprising a plurality of elementarypatterns which are stacked on one another, each elementary patternhaving an electrooptical material which has a high electroopticalcoefficient with respect to the read radiation and which is transparentto said read radiation and a first n-type photoconductive material and asecond p-type photoconductive material which surround the electroopticalmaterial and which are transparent to the read radiation and which,under the effect of the write radiation, are able to respectivelyproduce electrons having a high mobility of at least 10 cm² /V·S andholes having a high mobility of at least 10 cm² /V·S, saidelectrooptical coefficient being at least equal to 500 pm/V. 10.Apparatus according to claim 9, characterized in that each elementarypattern is separated from an adjacent elementary pattern by aninsulating layer, said insulating layer being transparent to the readradiation and constituting an electrical screen between the respectivephotoconductive materials of said elementary patterns.
 11. Apparatusaccording to claim 9, wherein the first photoconductive material is asemiconductor transparent to the read radiation and having, in aforbidden band of said first photoconductive material, a partiallyoccupied energy level which, under the effect of the write radiation,mainly emits electrons having a high mobility of at least 10 cm² /V·S,towards the conduction band of said semiconductor, and wherein thesecond photoconductive material is a semiconductor transparent to theread radiation and having, in a forbidden band of said secondphotoconducting material, a partially occupied energy level which, underthe effect of the write radiation, mainly emits holes having a highmobility of at least 10 cm² /V·S, towards the valence band of saidsemiconductor constituting the second photoconductive material. 12.Apparatus according to claim 11, wherein the energy levels correspondingrespectively to the first and the second photoconductive materials aredeep levels.
 13. Apparatus according to claim 11, wherein the first andthe second photoconductive materials are formed from the samesemiconductor material, whose doping is of the n type for the firstphotoconductive material and of the p type for the secondphotoconductive material.
 14. Apparatus according to claim 9, whereinthe thickness of the electrooptical material is an order of magnitudewhich is the same as the order of magnitude as the spacing of theinterference fringes.
 15. Apparatus according to claim 9, wherein thethickness of the electrooptical material is approximately 0.05 to 1micrometer.
 16. Apparatus according to claim 9, wherein theelectrooptical material has a multiple quantum well structure. 17.Apparatus according to claim 9, wherein the electrooptical material isin the form of a single layer.